Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.

The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz,
which falls into 18m HAM band when multiplied by two. I assume 27MHz
XTALs are 3rd overtone.

Since the circuitry is/will be digital, the oscillator is one gate of a
74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL.
10pF and 33pF (from the junkbox) capacitors to ground on each side of
the XTAL.

No oscillation (I have no oscilloscope, I use my HF receiver to
troubleshoot oscillators at known frequencies).

If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and
clear.

I have never built something with an overtone XTAL. I know that I need
an output resonating circuit if I want to extract the 3rd harmonic. Do I
need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW

PaoloC ) writes:
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.

The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz,
which falls into 18m HAM band when multiplied by two. I assume 27MHz
XTALs are 3rd overtone.

Since the circuitry is/will be digital, the oscillator is one gate of a
74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL.
10pF and 33pF (from the junkbox) capacitors to ground on each side of
the XTAL.

No oscillation (I have no oscilloscope, I use my HF receiver to
troubleshoot oscillators at known frequencies).

If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and
clear.

I have never built something with an overtone XTAL. I know that I need
an output resonating circuit if I want to extract the 3rd harmonic. Do I
need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW

Could the crystal be a receive crystal? CB crystals tended to show
the channel number or the frequency of the channel, and so if you simply
looked at the marked frequency, it would not tell you if it's for
transmit or receive. Though the ones I've seen did mark them with
"R" or "T". The point is that if it's a receive crystal, it wouldn't
be 1/3 of 27.125 but 27.125-IF and then divided by three.

I'd say there shouldn't be any problem getting an overtone crystal to
oscillate on the fundamental; if there's any problem it's getting an
overtone crystal to oscillate on its overtone frequency. Of course,
they usually oscillate at a slightly different frequency, so maybe
you need to tune around a bit.

Or maybe the circuit values are wrong for that crystal or frequency.
Or the crystal is indeed dead.

Michael VE2BVW

PaoloC ) writes:
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.

The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz,
which falls into 18m HAM band when multiplied by two. I assume 27MHz
XTALs are 3rd overtone.

Since the circuitry is/will be digital, the oscillator is one gate of a
74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL.
10pF and 33pF (from the junkbox) capacitors to ground on each side of
the XTAL.

No oscillation (I have no oscilloscope, I use my HF receiver to
troubleshoot oscillators at known frequencies).

If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and
clear.

I have never built something with an overtone XTAL. I know that I need
an output resonating circuit if I want to extract the 3rd harmonic. Do I
need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW

Could the crystal be a receive crystal? CB crystals tended to show
the channel number or the frequency of the channel, and so if you simply
looked at the marked frequency, it would not tell you if it's for
transmit or receive. Though the ones I've seen did mark them with
"R" or "T". The point is that if it's a receive crystal, it wouldn't
be 1/3 of 27.125 but 27.125-IF and then divided by three.

I'd say there shouldn't be any problem getting an overtone crystal to
oscillate on the fundamental; if there's any problem it's getting an
overtone crystal to oscillate on its overtone frequency. Of course,
they usually oscillate at a slightly different frequency, so maybe
you need to tune around a bit.

Or maybe the circuit values are wrong for that crystal or frequency.
Or the crystal is indeed dead.

Michael VE2BVW

PaoloC wrote in message ...
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.


need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW
Yup They are 3rd overtone. Try a little series inductance with the
rock, and maybe a few pf of capacitance in parallel. I seem to
remember these being cut for a 25 pf load versus the more common 330
or 32 pf load of fundamental rocks, but the rock may also be defective
(I've seen quite a few) Good luck, Mike.

PaoloC wrote in message ...
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.


need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW
Yup They are 3rd overtone. Try a little series inductance with the
rock, and maybe a few pf of capacitance in parallel. I seem to
remember these being cut for a 25 pf load versus the more common 330
or 32 pf load of fundamental rocks, but the rock may also be defective
(I've seen quite a few) Good luck, Mike.

Yup, they are 3rd. Try a little series inductance or parallel
capacitance. A few pf of cap. may do the trick. Basically these were
usually cut for 25 pf load versus the usual 32 or so of fundamental
rocks. BTW the fundamental will not be at exactly one third of the
overtone freq. (I don't remember why) You may very well have a bad
xtal too, it happens quite often. Good Luck, Mike

Yup, they are 3rd. Try a little series inductance or parallel
capacitance. A few pf of cap. may do the trick. Basically these were
usually cut for 25 pf load versus the usual 32 or so of fundamental
rocks. BTW the fundamental will not be at exactly one third of the
overtone freq. (I don't remember why) You may very well have a bad
xtal too, it happens quite often. Good Luck, Mike

Can't quite tell from your description what your circuit is, but if I recall
correctly back when i was doing hardware design a crystal oscillator using
logic gates required two gates. I used to use three---two for the
oscillator and the third for a buffer.

The two inverters where tied together with a capacitor between them. Then
the crystal was placed from the input of the first gate to the output of the
second. The output of the second also went to the buffer stage.


!-----------------crystal------------|
Like this: - gate one---capacitor---gate two---gate three---circuit
output

As I recall, there was also a resistor from the input of gate one to ground,
and from the output of gate two to ground.

I do not recall the capacitor or resistor values, but when properly built
this circuit never failed to oscillate with any crystal within the range of
the gates (fundamental mode only).

Jim
N8EE
"PaoloC" wrote in message
...
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.

The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz,
which falls into 18m HAM band when multiplied by two. I assume 27MHz
XTALs are 3rd overtone.

Since the circuitry is/will be digital, the oscillator is one gate of a
74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL.
10pF and 33pF (from the junkbox) capacitors to ground on each side of
the XTAL.

No oscillation (I have no oscilloscope, I use my HF receiver to
troubleshoot oscillators at known frequencies).

If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and
clear.

I have never built something with an overtone XTAL. I know that I need
an output resonating circuit if I want to extract the 3rd harmonic. Do I
need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW

Can't quite tell from your description what your circuit is, but if I recall
correctly back when i was doing hardware design a crystal oscillator using
logic gates required two gates. I used to use three---two for the
oscillator and the third for a buffer.

The two inverters where tied together with a capacitor between them. Then
the crystal was placed from the input of the first gate to the output of the
second. The output of the second also went to the buffer stage.


!-----------------crystal------------|
Like this: - gate one---capacitor---gate two---gate three---circuit
output

As I recall, there was also a resistor from the input of gate one to ground,
and from the output of gate two to ground.

I do not recall the capacitor or resistor values, but when properly built
this circuit never failed to oscillate with any crystal within the range of
the gates (fundamental mode only).

Jim
N8EE
"PaoloC" wrote in message
...
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.

The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz,
which falls into 18m HAM band when multiplied by two. I assume 27MHz
XTALs are 3rd overtone.

Since the circuitry is/will be digital, the oscillator is one gate of a
74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL.
10pF and 33pF (from the junkbox) capacitors to ground on each side of
the XTAL.

No oscillation (I have no oscilloscope, I use my HF receiver to
troubleshoot oscillators at known frequencies).

If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and
clear.

I have never built something with an overtone XTAL. I know that I need
an output resonating circuit if I want to extract the 3rd harmonic. Do I
need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW

"JLB" ) writes:
Can't quite tell from your description what your circuit is, but if I recall
correctly back when i was doing hardware design a crystal oscillator using
logic gates required two gates. I used to use three---two for the
oscillator and the third for a buffer.

The two inverters where tied together with a capacitor between them. Then
the crystal was placed from the input of the first gate to the output of the
second. The output of the second also went to the buffer stage.


!-----------------crystal------------|
Like this: - gate one---capacitor---gate two---gate three---circuit
output

As I recall, there was also a resistor from the input of gate one to ground,
and from the output of gate two to ground.

I do not recall the capacitor or resistor values, but when properly built
this circuit never failed to oscillate with any crystal within the range of
the gates (fundamental mode only).

Jim
N8EE

That was a standard oscillator with TTL. I used a 7400, and 470ohm resistors
from input to output of each gate. No coupling capacitor was needed, though
there was a low value cap in series with the crystal for load capacitance.
(The resistors linearized the gates.)

There were obviously variations on the theme, and your suggestion was
one of them.

And as you say, these tended to oscillate with most crystals. I had one
built up as a module, so I'd have a general purpose oscillator read,
to check crystals or as a signal source.

But, for some reason, when they moved to CMOS oscillators, the norm
became one gate, with a feedback resistor and a pair of capacitors, one
from input to ground, and the other from output to ground.

Michael VE2BVW

"PaoloC" wrote in message
...
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.

The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz,
which falls into 18m HAM band when multiplied by two. I assume 27MHz
XTALs are 3rd overtone.

Since the circuitry is/will be digital, the oscillator is one gate of a
74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL.
10pF and 33pF (from the junkbox) capacitors to ground on each side of
the XTAL.

No oscillation (I have no oscilloscope, I use my HF receiver to
troubleshoot oscillators at known frequencies).

If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and
clear.

I have never built something with an overtone XTAL. I know that I need
an output resonating circuit if I want to extract the 3rd harmonic. Do I
need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW